如何将DBNULL转换为双精度？

Question

即时获取数据库中的值。它出现了一个错误，提示“从DBNULL到Double的转换无效”。任何人，请帮助我？

公共职能共有（）为双 昏暗总为双

Dim strConn As String 
    strConn = ConfigurationManager.ConnectionStrings("***").ToString 
    Dim conn As New SqlConnection(strConn) 

    Dim strSql As String 
    strSql = "SELECT SUM (p.Price * c.Quantity) as 'Total' " & _ 
    "FROM CartItem sci INNER JOIN Product p ON c.ProductID=p.ProductID " & _ 
    "WHERE c.CartID=@CartID " 

    Dim cmd As New SqlCommand(strSql, conn) 

    cmd.Parameters.AddWithValue("@CartID", CartID) 

    Dim da As New SqlDataAdapter(cmd) 

    Dim ds As New DataSet 

    conn.Open() 

    da.Fill(ds, "CartItem") 

    conn.Close() 

    If (ds.Tables("CartItem").Rows.Count) <> 0 Then 
     **Total = ds.Tables("ShopCartItem").Rows(0)("Total")** 
    Else 
     Total = 0.0 
    End If 
    Return Total 
End Function 

Answer 1

试着改变你的SQL语句：

SELECT SUM (ISNULL(p.Price,0) * ISNULL(c.Quantity,0)) as 'Total' " & _ 
    "FROM CartItem sci INNER JOIN Product p ON c.ProductID=p.ProductID " & _ 
    "WHERE c.CartID=@CartID 

您还可以设置你的数据库表不允许为空

Answer 2

如果值可能为空，则返回类型需要为双倍？ （可空双精度）。

Answer 3

在设置之前，您需要检查 ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value。

If (ds.Tables("CartItem").Rows.Count) <> 0 Then 
    If ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value 
     Total = 0.0 
    else 
     Total = ds.Tables("ShopCartItem").Rows(0)("Total")** 
Else 
    Total = 0.0 
End If 

Answer 4

你可以改为使用Nullable（双）作为你的VB数据类型？

Answer 5

您必须检查是否Nullable(of Double)。 HasValue。 总= 0.0

If (ds.Tables("CartItem").Rows.Count) <> 0 Then 
    Dim sciValue As Nullable(Of Double) = DirectCast(ds.Tables("ShopCartItem").Rows(0)("Total"), Nullable(Of Double)) 
    If sciValue.HasValue Then 
     Total = sciValue.Value 
    End If 
End If 

Answer 6

你不能施放的DBNull到两倍。你可以，但是，它将它转换为可以为空的双（c＃double？）。

我写了一堆DataRow扩展方法（C＃），对此有很多帮助。使语法更加整齐。用法很简单。 A C＃示例：

public class Hormone 
{ 
    public int   ID    { get ; private set ; } 
    public HormoneLuType Type    { get ; private set ; } 
    public int   AgeStarted  { get ; private set ; } 
    public int   AgeStopped  { get ; private set ; } 
    public int   DurationInMonths { get ; private set ; } 
    public bool   IsCurrentlyUsing { get ; private set ; } 
    public DateTime?  DateLastEdited { get ; private set ; } 

    public string Name { get { return Type.ToString() } } 

    public Hormone(DataRow dr) 
    { 
     this.ID    =     dr.CastAsInt(    "ihormoneid")  ; 
     this.Type    = new HormoneLuType(dr.CastAsIntNullable(  "ihormluid" ) ?? 0) ; 
     this.AgeStarted  = (int)    dr.CastAsDecimal(   "nstartage" )  ; 
     this.AgeStopped  = (int)    dr.CastAsDecimal(   "nendage" )  ; 
     this.DurationInMonths = (int)    dr.CastAsDecimal(   "nduration" )  ; 
     this.IsCurrentlyUsing =     dr.CastAsBool(   "lusingnow" )  ; 
     this.DateLastEdited =     dr.CastAsDateTimeNullable("tedit"  )  ; 

     return ; 
    } 
} 

这里的扩展类：

using System; 
using System.Data; 

namespace DataAccess.Utils 
{ 
    public static class DataRowExtensions 
    { 

     #region downcast to DateTime 

     public static DateTime CastAsDateTime(this DataRow row , int index) 
     { 
      return toDateTime(row[index]) ; 
     } 
     public static DateTime CastAsDateTime(this DataRow row , string columnName) 
     { 
      return toDateTime(row[columnName]) ; 
     } 

     public static DateTime? CastAsDateTimeNullable(this DataRow row , int index) 
     { 
      return toDateTimeNullable(row[index]); 
     } 
     public static DateTime? CastAsDateTimeNullable(this DataRow row , string columnName) 
     { 
      return toDateTimeNullable(row[columnName]) ; 
     } 

     #region conversion helpers 

     private static DateTime toDateTime(object o) 
     { 
      DateTime value = (DateTime)o; 
      return value; 
     } 

     private static DateTime? toDateTimeNullable(object o) 
     { 
      bool hasValue = !(o is DBNull); 
      DateTime? value = (hasValue ? (DateTime?) o : (DateTime?) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to DateTime 

     #region downcast to byte[] 

     public static byte[] CastAsByteArray(this DataRow row , int index) 
     { 
      return toByteArray(row[index]); 
     } 
     public static byte[] CastAsByteArray(this DataRow row , string columnName) 
     { 
      return toByteArray(row[columnName]); 
     } 

     #region conversion helpers 

     private static byte[] toByteArray(object o) 
     { 
      bool hasValue = !(o is DBNull); 
      byte[] value = (hasValue ? (byte[]) o : (byte[]) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to Byte[] 

     #region downcast to int 

     public static int CastAsInt(this DataRow row , int index) 
     { 
      return toInt(row[index]) ; 
     } 
     public static int CastAsInt(this DataRow row , string columnName) 
     { 
      return toInt(row[columnName]) ; 
     } 

     public static int? CastAsIntNullable(this DataRow row , int index) 
     { 
      return toIntNullable(row[index]); 
     } 
     public static int? CastAsIntNullable(this DataRow row , string columnName) 
     { 
      return toIntNullable(row[columnName]) ; 
     } 

     #region conversion helpers 

     private static int toInt(object o) 
     { 
      int value = (int)o; 
      return value; 
     } 

     private static int? toIntNullable(object o) 
     { 
      bool hasValue = !(o is DBNull); 
      int? value = (hasValue ? (int?) o : (int?) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to int 

     #region downcast to int 

     public static decimal CastAsDecimal(this DataRow row , int index) 
     { 
      return toDecimal(row[index]) ; 
     } 
     public static decimal CastAsDecimal(this DataRow row , string columnName) 
     { 
      return toDecimal(row[columnName]) ; 
     } 

     public static decimal? CastAsDecimalNullable(this DataRow row , int index) 
     { 
      return toDecimalNullable(row[index]); 
     } 
     public static decimal? CastAsDecimalNullable(this DataRow row , string columnName) 
     { 
      return toDecimalNullable(row[columnName]) ; 
     } 

     #region conversion helpers 

     private static decimal toDecimal(object o) 
     { 
      decimal value = (decimal)o; 
      return value; 
     } 

     private static decimal? toDecimalNullable(object o) 
     { 
      bool  hasValue = !(o is DBNull); 
      decimal? value = (hasValue ? (decimal?) o : (decimal?) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to int 

     #region downcast to bool 

     public static bool CastAsBool(this DataRow row , int index) 
     { 
      return toBool(row[index]) ; 
     } 
     public static bool CastAsBool(this DataRow row , string columnName) 
     { 
      return toBool(row[columnName]) ; 
     } 

     public static bool? CastAsBoolNullable(this DataRow row , int index) 
     { 
      return toBoolNullable(row[index]); 
     } 
     public static bool? CastAsBoolNullable(this DataRow row , string columnName) 
     { 
      return toBoolNullable(row[columnName]) ; 
     } 

     #region conversion helpers 

     private static bool toBool(object o) 
     { 
      bool value = (bool)o; 
      return value; 
     } 

     private static bool? toBoolNullable(object o) 
     { 
      bool hasValue = !(o is DBNull); 
      bool? value = (hasValue ? (bool?) o : (bool?) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to bool 

     #region downcast to string 

     public static string CastAsString(this DataRow row , int index) 
     { 
      return toString(row[index]); 
     } 
     public static string CastAsString(this DataRow row , string columnName) 
     { 
      return toString(row[columnName]); 
     } 

     #region conversion helpers 

     private static string toString(object o) 
     { 
      bool hasValue = !(o is DBNull); 
      string value = (hasValue ? (string) o : (string) null) ; 
      return value; 
     } 

     #endregion 

     #endregion downcast to string 

    } 
} 

希望这有助于！